AA Comment on “The possible explanation of neutron lifetime beam anomaly” byA. P. Serebrov, et al.
F. E. Wietfeldt, R. Biswas, and R. W. Haun ∗ Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118
M. S. Dewey, C. C. Haddock, S. F. Hoogerheide, H. P. Mumm, and J. S. Nico
National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
J. Caylor and N. Fomin
Department of Physics, University of Tennessee, Knoxville, TN 37996, USA
G. L. Greene
Department of Physics, University of Tennessee, Knoxville, TN 37996, USA andPhysics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Physics Department, Gettysburg College, Gettysburg, PA 17325, USA
W. M. Snow
Physics Department, Indiana University, Bloomington, IN 47405, USA (Dated: April 3, 2020)
In a recent manuscript, Serebrov et al.  proposethat loss of protons due to residual gas interactions inthe most recent beam neutron lifetime experiment [2, 3]led to a systematic error that could account for the wellknown disagreement between the beam method [2–5] andthe ultracold neutron storage method [6–13]. In theirpaper, Serebrov et al. make a simpliﬁed model of thevacuum environment of the trap as a vessel with coldwalls (the magnet bore) located inside another vesselwith warm walls (the outer vacuum system). They as-sume that residual gas ﬂows from the outer vessel intothe inner vessel, remaining in gas phase at thermal equi-librium with the walls in the two vessels. Therefore themolecular density in the inner vessel reaches equilibriumat n = P/k √ T T , where P is the vacuum pressure inthe outer chamber, k is the Boltzmann constant, and T , T are the vessel temperatures. Using P = 10 − mbar asthe ion gauge pressure (actually the upper limit as thegauge was under range) and T = 300 K, T = 4 K, theyobtain n = 2 . × cm − inside the trap. Later theyshow that, at such a density, charge exchange by trappedprotons with residual gas components such as H O, CH ,CO, and CO would cause a signiﬁcant loss during the 10ms trap period and result in a measured neutron lifetimethat is too long. While residual gas interactions shouldoccur at some level, we ﬁnd this analysis to be ﬂawedbecause it neglects cryocondensation on the cold bore, acrucial feature of the trap vacuum.The cold bore of the magnet was a 45 cm long, 12 cminner diameter stainless steel tube in direct contact withthe liquid helium bath. Its operational temperature wasabout 8 K. At this temperature the condensation coeﬃ- cients of most gases are close to unity so residual gas willcondense on the wall after just a few collisions, ratherthan remain in the gas phase and reach thermal equilib-rium. The bore is eﬀectively a cryopump. According tothe theory of cryocondensation (see for example [14, 15])the partial pressure of each gas component in the borewill reach equilibrium close to its saturation vapor pres-sure. Figure 1 shows a plot of saturation vapor pressure vs. temperature for a number of common gases. Otherthan hydrogen, helium, and neon the partial pressure anddensity of all residual gas components are predicted to befar lower than the estimate in , although we note thatdetermining the actual partial pressures of species insidethe proton trap is a complicated problem that dependson many factors. There is no reason to expect neon in thevacuum system. One would expect hydrogen of course,and also helium due to its omnipresence in the guide hallatmosphere. Charge exchange with these species wouldresult in trapped hydrogen (monatomic or diatomic) andhelium ions that could be detected by the surface barrierdetector after the trap is opened.In summary, we ﬁnd the analysis of Serebrov et al. to be incorrect due to their neglect of cryocondensationof residual gas on the cold magnet bore that enclosesthe proton trap in the beam lifetime experiment. Moregenerally, for the past few years we have been actively in-vestigating many systematic eﬀects in the beam neutronlifetime experiment, including those that could be causedby residual gas and other vacuum related phenomena.This is primarily an experimental eﬀort, as the appara-tus is very complicated and diﬃcult to model accuratelyand to useful precision in a simulation or calculation. a r X i v : . [ nu c l - e x ] A p r Figure 3.
Boiling-point temperature of common gases as a function of external pressure.
Figure 3 expands on Fig. 2 by describing the dependence of boiling-point or sublimation tempera-ture on external pressure for common cryogens. Also noted is the triple point where the cryogentransitions to a solid. This plot also indicates the temperature and pressure where external contami-nant gases, such as water vapor, will begin to condense on cryogenic surfaces such as low-emit-tance shields and MLI. Preventing such condensation is a critical issue for managing radiant para-sitic loads on low-emittance shields and cryogenic surfaces. This topic of emittance degradationfrom contaminant films is covered later in this chapter in Section 18.104.22.168.
Over the years, many liquid cryogenic systems have been developed, fabricated, and operatedin both ground environments and in space. They cover a wide range of cryogen fluids and construc-tion features in terms of stored volume, pressure and temperature limitations, and relative effi-ciency in terms of the parasitic heat leaks. Many of these systems utilize liquid helium for achiev-ing temperatures between 1.4 K and 4 K or liquid nitrogen for achieving temperatures around 77 K.To achieve temperatures below 4.2 K requires that liquid helium be stored under partial vacuumconditions. At pressures from 10 to 40 torr, temperatures in the range of 1.4 K to 1.8 K are achiev-able with liquid helium.
Typical Dewar Construction Features.
As illustrated in Fig. 4, liquid cryogen systems typi-cally involve a nested storage tank concept whereby the inner tank, which holds the liquid cryogen,is suspended inside an outer vacuum shell with low-conductivity structural supports. These struc-tural supports are typically made of low-conductivity tubes, struts or tension bands in order toachieve high structural efficiency and minimum conductivity between the two tanks. The gapbetween the two tanks is then evacuated and filled with Multilayer Insulation (MLI). In addition, ahigh efficiency dewar may also contain one or more strategically placed vapor-cooled shields (VCS)that are cooled by the evaporating cryogen as it vents from the inner tank.The goal of the gap construction is to prevent gaseous conduction and radiation between theouter an inner tank and to achieve maximum thermal benefit from the evaporating cryogen. Al-though the heat of vaporization of the cryogen is the primary cooling force in the system, there isalso considerable benefit associated with extracting the available heat from the vapor as it rises upin temperature from the cryogen temperature to the external vent temperature. This is accom-plished by piping the venting gas through the vapor cooled shields, which serve to intercept muchof the radiant energy coming through the MLI layers from the outer tank. The VCS can also beattached to the support struts or plumbing to further reduce conductive heat leaks.
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Low Temperature Materials and Mecha-nisms , ed. by Y. Bar-Cohen, pp. 109-181, CRC Press,USA (2019), ISBN 0-367-87134-3. ∗∗